1. Field of the Invention
The invention relates to a method for forming micropatterns, and in particular, it relates to a method for forming micropatterns necessary for producing an optical disk master for manufacturing optical disks and the like for recording information at high density.
2. Description of the Related Art
Recently, to realize optical disks with higher density, narrower track pitches are employed for the guide grooves and pre-pits of optical disks. The guide grooves and the pre-pits are generally formed by a so-called mastering process; i.e., the optical disk master is produced by exposure and development of a photoresist, which comprises irradiating a converged laser radiation to the photoresist coated on a glass substrate.
In this case, the optical beam spot diameter of the converged laser radiation is approximately 0.8 λ/NA, where λ represents the wavelength of the laser radiation, and NA represents the numerical aperture of the objective lens for converging the laser radiation.
Conventionally, in order to realize narrower track pitches for the guide grooves and pre-pits on optical disks, the wavelength λ of the laser radiation is shortened and the numerical aperture NA of the objective lens is increased with an aim to decrease the spot diameter of the optical beam.
Laser cutting conventionally employed for an optical disk master having coated thereon a positive type photoresist 6 is described below. FIG. 1 shows a schematically shown constitution of a conventional laser cutting.
Referring to FIG. 1, a laser radiation 2 emitted from a laser light source 1 is reflected by mirrors 3-1 and 3-2, and after the optical intensity is controlled by an optical modulator 4, the laser radiation is reflected by a down edge mirror 3-3 and is transmitted through an objective lens 5 to be convergent irradiated onto a positive-type photoresist coated on a glass substrate 7.
The glass substrate 7 is attached to a spindle motor 8. By moving the down edge mirror 3-3 and the objective lens 5 in synchronization with the rotation of the glass substrate 7 in accordance with the rotation of the spindle motor, exposure is performed on the positive-type photoresist 6 in correspondence with the spiral-like guide groove and pre-pits. After the exposure, positive-type photoresist patterns corresponding to the spiral-like guide groove and pre-pits are formed by carrying out the development of the positive-type photoresist 6.
In FIG. 2 is shown a normalized optical intensity distribution with respect to the spot diameter of the optical beam converged on a positive-type photoresist 6 of a conventional type. This shows an approximately Gaussian optical intensity distribution.
In general, the optical beam spot diameter BS is defined by the range in which the optical intensity becomes 1/e2 of the maximum optical intensity. The optical beam spot diameter BS depends on the wavelength λ of the employed laser radiation 2 and the numerical aperture NA of the objective lens 5 for converging the laser radiation 2, and is approximated by ca. 0.8×λ/NA.
For instance, in case of using a Kr laser light source 1 with a laser radiation 351 nm in wavelength as the laser radiation 2, and by using an objective lens having a numerical aperture NA of 0.95, the optical beam spot diameter BS becomes 296 nm.
FIG. 3 shows the state in which latent image 9 is formed in case the positive-type photoresist 6 formed on a glass substrate 7 is exposed to an optical beam 2 having the optical beam spot diameter BS described above. On passing through the positive-type photoresist 6, the optical intensity of the optical beam 2 becomes weaker due to optical absorption, as to form a wide latent image 9 on the surface of the positive-type photoresist despite it is narrow on the glass substrate.
In FIG. 4 is shown the state in case the latent image 9 is formed by exposing neighboring guide groove with a track pitch TP approximately equal to the optical beam spot diameter BS. For instance, in case the optical beam spot diameter BS is 296 nm, the track pitch TP is 300 nm. The position of the latent image 9 corresponds to the guide groove.
In FIG. 5 is shown the state in case the latent image 9 is formed on the positive-type photoresist 6 after continuously forming spiral-like guide groove above. FIG. 6 gives a positive-type photoresist pattern 10 obtained after developing the latent image 9 shown in FIG. 5.
Referring to FIG. 6, since the optical beam spot diameter BS is approximately the same as the track pitch TP, only slight positive-type photoresist pattern 10 remains between the guide grooves 11, and, furthermore, it has been found that rectangular patterns cannot be obtained. In such a state, a slight change in optical beam intensity on cutting or a fluctuation in track pitch attributed to external oscillation considerably affects the shape of the positive-type photoresist pattern 10, and in a worst case, it has been observed to generate drop outs of the positive-type photoresist pattern 10 to make stable tracking difficult.
To circumvent such inconveniences, a wider positive-type photoresist pattern 10 is found necessary. Accordingly, an attempt has been made to form a wider positive-type photoresist pattern 10 by decreasing the intensity of the laser radiation 2 in exposing the positive-type photoresist 6.
In FIG. 7 is shown the state of a latent image obtained with a laser radiation decreased in intensity. Referring to FIG. 7, with a laser radiation 2 decreased in intensity in exposing, a V shaped groove-like latent image 9 corresponding to the optical intensity distribution of the optical beam spot is found to form, and in this case again, it has been confirmed that no rectangular positive-type photoresist pattern is formed.
Furthermore, a track pitch TP greater than, about twice, the optical beam spot diameter BS, is necessary to obtain a rectangular pattern.
Accordingly, in case a glass substrate having coated directly thereon a positive-type photoresist 6 is used for the production of an optical disk master, it has been found difficult to obtain a narrowed track pitch while retaining stable tracking performance.
Furthermore, the numerical aperture NA of the objective lens used at present is already approaching the limit, and, since laser radiation in the ultraviolet region is currently in use, it is difficult to use a laser radiation shorter in wavelength. More specifically, an objective lens with a numerical aperture NA of 0.95 is employed, and a Kr laser 351 nm in wavelength is utilized as the light source. In this case, the resulting optical beam spot diameter is about 0.3 μm, and it is unfeasible to realize a track pitch of 0.3 μm or shorter.